The exhaust gas discharged from a vehicle using gasoline as fuel contains harmful components such as hydrocarbons (THC), carbon monoxide (CO), and nitrogen oxides (NOx). Hence, it is required to purify each of the harmful components so as to convert the hydrocarbons (THC) into water and carbon dioxide through oxidation, carbon monoxide (CO) into carbon dioxide through oxidation, and the nitrogen oxides (NOx) into nitrogen through reduction.
As the catalyst (hereinafter, referred to as the “exhaust gas purification catalyst”) for treating such exhaust gas, a three way catalyst (TWC) which is able to oxidize or reduce CO, THC, and NOx is used.
As such a three way catalyst, one that is obtained by supporting a precious metal on an oxide porous material having a high specific surface area, for example, an alumina porous material having a high specific surface area and supporting this on a substrate, for example, a monolithic substrate having a refractory ceramic or metallic honeycomb structure or on refractory particles is known.
Meanwhile, the exhaust gas discharged from a diesel engine contains sulfates derived from the sulfur component in the fuel and tar-like fine particulate materials (referred to as “PM”) due to incomplete combustion, and the like in addition to CO, THC, and NOx described above.
As a device for removing CO and THC contained in the exhaust gas discharged from a diesel engine, a diesel oxidation catalyst (referred to as “DOC”) is known.
As DOC, one that is obtained by coating a refractory inorganic porous material such as zeolite or Al2O3 on a porous filter substrate having a honeycomb structure is known.
A precious metal such as platinum (Pt), palladium (Pd), or rhodium (Rh) is often used in both of the catalysts that are a catalyst for purifying the exhaust gas discharged from a gasoline engine and a catalyst for purifying the exhaust gas discharged from a diesel engine as a catalytically active component. Moreover, the bonding strength between these precious metals as a catalytically active component and the substrate is not so strong and the specific surface area of the substrate itself is not also so great, and it is thus difficult to support the precious metal on the substrate in a sufficient supporting amount and a highly dispersed manner even when it is attempted to directly support the precious metal on the substrate. Hence, it is general to support a precious metal on a particulate catalyst carrier having a high specific surface area in order to support a sufficient amount of a catalytically active component on the surface of a substrate in a highly dispersed manner.
As this kind of carrier for exhaust gas purification catalyst (also referred to as the “catalyst carrier” or “carrier”), porous particles composed of refractory inorganic oxides such as silica, alumina, and titania compounds are now widely used.
The purification properties (ternary properties) of CO, HC, and NOx by the three way catalyst are closely related to the stoichiometric air-fuel ratio (A/F) which indicates the atmosphere of the exhaust gas discharged from a motor vehicle, and it has thus been general in the prior art to control the purification properties to the condition of being mainly A/F=14.6 (stoichiometric air-fuel ratio) in which the exhaust gas purifying function is sufficiently exerted, namely, the condition of being in the vicinity of an excess air ratio λ=1.
In recent years, the control of engine at an air-fuel ratio higher than the stoichiometric air-fuel ratio (A/F=14.6), namely, under a lean condition of about 14.6<A/F≦16.0 has been desired from the viewpoint of the improvement of fuel consumption and the cutback of carbon dioxide emissions. It is possible to suppress the generation of CO2 of the combustion exhaust gas as well as the fuel consumption is improved since the amount of fuel used decreases if the internal combustion engine is driven under a lean condition to cause lean burn in an oxygen-rich atmosphere.
However, under the lean condition as described above, oxygen is excessive and the NOx purification performance by the exhaust gas purification catalyst significantly deteriorates, and the development of a catalyst capable of purifying NOx at a high efficiency even under the lean condition has thus been desired.
Accordingly, a metal phosphate has attracted attention as a catalyst carrier which exhibits excellent NOx purification performance under the lean condition and also excellent sulfur poisoning resistance.
For example, an exhaust gas purification catalyst in which one kind or two or more kinds of precious metals selected from the group consisting of Pt, Pd, Rh, and Ir are supported on a phosphoric acid compound of aluminum phosphate, zirconium phosphate, or silicoaluminophosphate at from 0.01 to 5 wt % is disclosed in Patent Document 1 (JP 8-150339 A).
A denitrification catalyst obtained by supporting iridium as an active metal on a carrier composed of at least one or more kinds of compounds selected from a metal carbonate, a metal sulfate, and a metal phosphate is disclosed in Patent Document 2 (JP 11-267509 A).
An exhaust gas purification catalyst which is used for purifying NOx in the exhaust gas that is discharged from an internal combustion engine and contains oxygen in an excess amount and constituted by a carrier which includes an anion portion composed of a compound containing phosphorus oxide or sulfur oxide and a cation portion for compensating a charge and has a high solid acid strength and a precious metal element supported on this carrier is disclosed in Patent Document 3 (JP 2010-440 A).
In Patent Document 4 (JP 2013-252465A), a catalyst carrier for exhaust gas purification containing a phosphate represented by a general formula, MPO4 (in the formula, M is Y, La, or Al) or zirconium phosphate represented by a formula, ZrP2O7 is disclosed as a catalyst carrier for exhaust gas purification which suppresses a decrease in NOx purification activity in a lean region in which the excess air ratio λ is greater than 1 and can significantly improve the performance as compared to Rh-supporting zirconia, and a catalyst structure for exhaust gas purification including a catalyst for exhaust gas purification containing a precious metal which includes at least Rh and is supported on the carrier, a catalyst support composed of a ceramic or a metal material, and a layer of the catalyst for exhaust gas purification that is supported on the catalyst support is disclosed.